In undersea cables for transporting power, up to a recent date, the conductors were insulated by means of paper strips impregnated with viscous substances such as grease or petroleum tar. The interstices between the conductors were then filled with packing substances also made of viscous substances.
When impregnated paper tape insulant was replaced by polyethylene insulant, it was observed that the packing substances formed by viscous substances melted when the insulant was installed by extrusion and migrated into the insulant, thereby considerably disturbing its dielectric properties.
Les Cables de Lyon's published French patent specification No. 2 407 557 dated Oct. 27, 1977, recommends substituting rods of plastics substances in which cellulose fibres are fixed for a packing substance between a polyethylene insulant and a screen with longitudinal corrugations. Said fibres swell when the rods are placed in water and thereby prevent water propagating. However, it is not possible to assemble such rods with metal conductors in a die.
Preferred embodiments of the present invention mitigate these draw-backs and provide a semiconductor packing substance for an undersea cable, which substance provides just as good sealing against the longitudinal propagation of water as previously known viscous substances while not melting when the insulant is installed by extrusion nor disturbing the dielectric properties thereof.